Have you actually clicked on their names to see if they're members or not? If someone joined today, the 'M' won't show up by their name immediately. Takes a bit. Best way to see if someone is a member, if you're concerned, is to click on their name and on the ensuing screen you'll see whether or not they're a member.

Seriously. The member non/member dynamic on this forum is laughable. Paying for this site is a waste of money and tattling on people who are trying to sell items so they can make a few bucks in this economy is a complete waste of time and character.